Air suspension systems have significant advantages over conventional steel suspension systems and are therefore increasingly being used both on commercial vehicles, such as heavy goods vehicles and buses, and on, preferably heavy, passenger vehicles, such as luxury cars and off-road vehicles. Thus, an air suspension system allows level control independent of loading since the current state of loading can be compensated in each case by adapting the bellows pressure in the air spring bellows. By virtue of the progressive spring characteristics of the air springs, an air suspension system likewise offers particularly reliable contact between the wheels and the roadway and comfortable response behavior during wheel compression and rebound. Another advantage of air suspension systems is that the ground clearance of the vehicles concerned can be modified when required, e.g. can be increased for off-road use and reduced for high-speed driving on freeways. In the case of commercial vehicles, there is the additional fact that the vehicle body can be lowered or adjusted to a suitable level for loading and unloading. Thus, for example, the vehicle chassis of an air-suspended heavy goods vehicle or trailer can be lowered to set down an interchangeable flatbed and raised again to pick it up. It is likewise possible for the load surface of a heavy goods vehicle to be adjusted to the level of a loading ramp to facilitate loading and unloading by lowering or raising the bellows pressure at the rear axle. In the case of air-suspended buses, the vehicle body can be lowered on one side by releasing the compressed air from the spring bellows on the outside of the roadway to make it easier for passengers to get in and out, and can then be raised again by filling the spring bellows. This function is also known as kneeling or easy entry.
To control the level of a corresponding motor vehicle by means of displacement sensors, which are arranged on both sides, in each case between the vehicle axles or suspension elements of the vehicle axles and the vehicle body, the respective actual level of the vehicle body in relation to the vehicle axle is detected and compared in a control unit with the predetermined setpoint level stored there. If the respective level control valves associated with each of the air spring bellows arranged on both sides on the vehicle axles, are, as envisaged in the present case, designed as switching valves, to be more precise as 2/2-way switching valves, level control is performed, as is known, by connecting the relevant air spring bellows to a pressurized main line by opening the associated level control valve if a lower setpoint level tolerance limit is undershot, and thereby supplying it with air, and connecting said bellows to the main line by opening the associated level control valve, said main line then being unpressurized, if an upper setpoint level tolerance limit is exceeded, and thereby releasing air from it. To ensure that the main line is pressurized or unpressurized, as required at any particular time, it is in each case connected before this, e.g. via a main switching valve designed as a 3/2-way switching valve, to a compressed air source, e.g. a pressure accumulator or the pressure line of a compressor, or to a compressed air sink, e.g. an unpressurized line connected via a muffler to the environment. Allowing for the two setpoint level tolerance limits avoids a situation where the air spring bellows are alternately supplied with air or relieved of air at the slightest deviation between the respective actual level and the predetermined setpoint level, which would reduce the comfort of the vehicle passengers and would furthermore be associated with increased compressed air consumption. The structure of a corresponding air suspension system and the operation of a level control system of this kind is described in EP 0 779 166 B1, for example.
Said tolerance limits for a permitted deviation of the actual level from the setpoint level are usually defined specifically for the vehicle and represent a compromise between safe driving and high driving comfort. However, it is also possible to modify these tolerance limits in a suitable manner if certain operating states arise. Thus, in DE 195 39 887 B4, for example, there is a description of a method for controlling the level of an air-suspended motor vehicle, in particular a heavy goods vehicle, in which either a time delay in the level control is activated or the control dead band of the level control is extended by raising the upper tolerance limit and lowering the lower tolerance limit if the motor vehicle cannot be moved. By temporarily switching off level control, this is intended to prevent correction of brief deviations in the actual level from the setpoint level at one vehicle axle during a loading operation, caused, for example, by a relatively heavy forklift truck driving onto and off the load surface. The relevant operating state of the motor vehicle can be identified from the fact that the engine is switched off.
EP 0 779 166 B1 discloses a method for controlling the level of an air-suspended motor vehicle, in which, if the vehicle body assumes a tilted position at one vehicle axle, the correction of the tilted position may be broken off. Correcting the tilted position requires supplying air to the air spring(s) on the side on which the vehicle body has been lowered and releasing air from the air spring(s) on the side on which the vehicle body has been raised. Since this is accomplished by alternately switching over the connection of the main line to the compressed air source and the compressed air sink, provision is made for the number of opposite control cycles to be detected and evaluated. If a predetermined number of opposite control cycles is exceeded, the setpoint levels of the relevant vehicle axle(s) are set to match the current actual level by correspondingly shifting the respective upper and lower tolerance limits. This is intended to avoid exceeding a maximum pressure difference in the air springs of a vehicle axle and undershooting a minimum pressure in one of the air springs without using pressure sensors.
EP 1 925 471 B1 furthermore describes a method for controlling the level of an air-suspended motor vehicle in which, while driving, the transverse acceleration of the motor vehicle is detected and the setpoint level tolerance limits for the air springs on the inside of a bend and on the outside of a bend are modified differently in accordance with the transverse acceleration. Thus, according to this method, it is envisaged that the upper tolerance limit of the air spring bellows on the inside of a bend is raised further than that of the air spring bellows on the outside of a bend as the transverse acceleration increases, and that the lower tolerance limit of the air spring bellows on the outside of a bend is lowered further than that of the air spring bellows on the inside of a bend as the transverse acceleration increases. Owing to the asymmetric shifting of the tolerance limits for the air springs on a vehicle axle, tilting of the vehicle body is counteracted and opposing control processes are avoided.
During the operation of a vehicle, it may happen that it comes to a halt at a point where the front right-hand wheel is in a pothole or some other depression in the road surface, for example, while all the other vehicle wheels are standing on a level surface. In this situation, the displacement sensor at the front right-hand wheel supplies a control unit of the level control system with a distance value or actual level which is greater than distance values or actual levels at the other vehicle wheels and exceeds a tolerance limit. When operating in a conventional mode, the control unit can draw the conclusion from this that the front right-hand wheel is in a pothole or the like, with the result that the control unit decides to correct this state on the basis of a control program stored therein. For this purpose, compressed air is passed into air spring bellows and possibly released from other air spring bellows according to a selected strategy. The system then measures whether the actual level values of the individual wheel positions are once again within the limits of a tolerance band around a setpoint level value, i.e. have approached the setpoint level in a predetermined way. If the result of this control operation is worse, air is released from the air spring bellows of the vehicle, or they are filled with additional compressed air, in some other way on the basis of the new distance or actual level values obtained by the displacement sensors at the different wheel positions. This control process can take an inconveniently long time and can require a relatively large number of switching valve actuations in the level control system, thereby using up the entire service life of such switching valves over a relatively short time overall.